轉自:http://www.2cto.com/kf/201501/368954.html
在項目中,都或多或少地使用的Tab佈局,所以大都會用到ViewPager+Fragment,但是Fragment有個不好或者太好的地方。例如你在ViewPager中添加了三個Fragment,當加載ViewPager中第一個Fragment時,它會默認幫你預先加載了第二個Fragment,當你加載第二個Fragment時,它會幫你加載第三個Fragment。這樣雖然有時很好,但是用戶只需看一個Fragment時,我們就做了一些多餘工作加載了第二個Fragment。在這只需要取消Fragment的預加載即可,只有當用戶切換到某個Fragment才加載..
首先,介紹兩個方法void
setUserVisibleHint(boolean isVisibleToUser)、boolean getUserVisibleHint(),它們分別用作設置/獲得Fragment可見狀態,我們可以重寫Fragment在其中做判斷,代碼如下:
import android.support.v4.app.Fragment;
public abstract class BaseFragment extends Fragment {
/** Fragment當前狀態是否可見 */
protected boolean isVisible;
@Override
public void setUserVisibleHint(boolean isVisibleToUser) {
super.setUserVisibleHint(isVisibleToUser);
if(getUserVisibleHint()) {
isVisible = true;
onVisible();
} else {
isVisible = false;
onInvisible();
}
}
/**
* 可見
*/
protected void onVisible() {
lazyLoad();
}
/**
* 不可見
*/
protected void onInvisible() {
}
/**
* 延遲加載
* 子類必須重寫此方法
*/
protected abstract void lazyLoad();
}
在我們的Fragment中,只需要繼承這個類,然後重寫其中的lazyLoad()方法,當Fragment對用戶可見(即用戶切換到此Fragment時)我們在lazyLoad()中加載所需數據,還有一點,我們需要知道fragment的setUserVisibleHint方法處在fragment生命週期的什麼階段,經測試該方法位於oncreate和onCreateView方法直接。 詳細代碼看下面,我寫了個假的獲取數據線程:
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.View;
import android.view.ViewGroup;
import android.widget.TextView;
public class CustomListFragment extends BaseFragment {
private static final String FRAGMENT_INDEX = fragment_index;
private final int FIRST_FRAGMENT = 0;
private final int SECOND_FRAGMENT = 1;
private final int THIRD_FRAGMENT = 2;
private TextView mFragmentView;
private int mCurIndex = -1;
/** 標誌位,標誌已經初始化完成 */
private boolean isPrepared;
/** 是否已被加載過一次,第二次就不再去請求數據了 */
private boolean mHasLoadedOnce;
/**
* 創建新實例
*
* @param index
* @return
*/
public static CustomListFragment newInstance(int index) {
Bundle bundle = new Bundle();
bundle.putInt(FRAGMENT_INDEX, index);
CustomListFragment fragment = new CustomListFragment();
fragment.setArguments(bundle);
return fragment;
}
@Override
public View onCreateView(LayoutInflater inflater, ViewGroup container, Bundle savedInstanceState) {
if(mFragmentView == null) {
mFragmentView = (TextView) inflater.inflate(R.layout.fragment, container, false);
//獲得索引值
Bundle bundle = getArguments();
if (bundle != null) {
mCurIndex = bundle.getInt(FRAGMENT_INDEX);
}
isPrepared = true;
lazyLoad();
}
//因爲共用一個Fragment視圖,所以當前這個視圖已被加載到Activity中,必須先清除後再加入Activity
ViewGroup parent = (ViewGroup)mFragmentView.getParent();
if(parent != null) {
parent.removeView(mFragmentView);
}
return mFragmentView;
}
@Override
protected void lazyLoad() {
if (!isPrepared || !isVisible || mHasLoadedOnce) {
return;
}
new AsyncTask<void, boolean="">() {
@Override
protected void onPreExecute() {
super.onPreExecute();
//顯示加載進度對話框
UIHelper.showDialogForLoading(getActivity(), 正在加載..., true);
}
@Override
protected Boolean doInBackground(Void... params) {
try {
Thread.sleep(2000);
//在這裏添加調用接口獲取數據的代碼
//doSomething()
} catch (Exception e) {
e.printStackTrace();
}
return true;
}
@Override
protected void onPostExecute(Boolean isSuccess) {
if (isSuccess) {
// 加載成功
setView();
mHasLoadedOnce = true;
} else {
// 加載失敗
}
//關閉對話框
UIHelper.hideDialogForLoading();
}
}.execute();
}
private void setView() {
// 根據索引加載不同視圖
switch (mCurIndex) {
case FIRST_FRAGMENT:
mFragmentView.setText(第一個);
break;
case SECOND_FRAGMENT:
mFragmentView.setText(第二個);
break;
case THIRD_FRAGMENT:
mFragmentView.setText(第三個);
break;
}
}
}
到這裏我們只是寫好了Fragment,在FragmentActivity中還需要對ViewPager設置一下,讓它每次只加載一個Fragment,ViewPager.setOffscreenPageLimit(int limit),其中參數可以設爲0或者1,參數小於1時,會默認用1來作爲參數,未設置之前,ViewPager會默認加載兩個Fragment。所以,我們只需要調用下它,設置下加載Fragment個數即可。
import java.util.ArrayList;
import java.util.List;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentActivity;
import android.support.v4.view.ViewPager;
import android.support.v4.view.ViewPager.OnPageChangeListener;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.RadioButton;
import android.os.Bundle;
public class MainActivity extends FragmentActivity implements OnClickListener{
private RadioButton mFstBtn;
private RadioButton mSndBtn;
private RadioButton mThdBtn;
private ViewPager mViewPager;
private ListFragmentPagerAdapter mPagerAdapter;
private List<fragment> mFragments = new ArrayList<fragment>();
private final int FIRST_FRAGMENT = 0;
private final int SECOND_FRAGMENT = 1;
private final int THIRD_FRAGMENT = 2;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
initButton();
initViewPager();
}
/**
* 初始化按鈕
*/
private void initButton() {
mFstBtn = (RadioButton)findViewById(R.id.id_rb_fst);
mFstBtn.setOnClickListener(this);
mSndBtn = (RadioButton)findViewById(R.id.id_rb_snd);
mSndBtn.setOnClickListener(this);
mThdBtn = (RadioButton)findViewById(R.id.id_rb_thd);
mThdBtn.setOnClickListener(this);
}
/**
* 初始化ViewPager控件
*/
private void initViewPager() {
mViewPager = (ViewPager)findViewById(R.id.id_vp_viewpager);
//關閉預加載,默認一次只加載一個Fragment
mViewPager.setOffscreenPageLimit(1);
//添加Fragment
mFragments.add(CustomListFragment.newInstance(FIRST_FRAGMENT));
mFragments.add(CustomListFragment.newInstance(SECOND_FRAGMENT));
mFragments.add(CustomListFragment.newInstance(THIRD_FRAGMENT));
//適配器
mPagerAdapter = new ListFragmentPagerAdapter(getSupportFragmentManager(), mFragments);
mViewPager.setAdapter(mPagerAdapter);
mViewPager.setOnPageChangeListener(onPageChangeListener);
}
private OnPageChangeListener onPageChangeListener = new OnPageChangeListener() {
@Override
public void onPageSelected(int position) {
//根據用戶選中的按鈕修改按鈕樣式
switch (position) {
case FIRST_FRAGMENT:
mFstBtn.setChecked(true);
mSndBtn.setChecked(false);
mThdBtn.setChecked(false);
break;
case SECOND_FRAGMENT:
mFstBtn.setChecked(false);
mSndBtn.setChecked(true);
mThdBtn.setChecked(false);
break;
case THIRD_FRAGMENT:
mFstBtn.setChecked(false);
mSndBtn.setChecked(false);
mThdBtn.setChecked(true);
break;
}
}
@Override
public void onPageScrolled(int arg0, float arg1, int arg2) {}
@Override
public void onPageScrollStateChanged(int arg0) {}
};
@Override
public void onClick(View v) {
switch (v.getId()) {
case R.id.id_rb_fst:
mViewPager.setCurrentItem(FIRST_FRAGMENT);
break;
case R.id.id_rb_snd:
mViewPager.setCurrentItem(SECOND_FRAGMENT);
break;
case R.id.id_rb_thd:
mViewPager.setCurrentItem(THIRD_FRAGMENT);
break;
}
}
}